This invention relates generally to the measurement of radioactivity of a sample in a container and, more particularly, to a new and improved electronic method and apparatus for quantitatively measuring the radioactivity of an individual sample in a laboratory environment.
In the biological and medical sciences, certain radioisotopes are frequently used as tracers in tests and experiments in order to detect minute quantities of certain biochemicals present in test samples. For example, the radioisotope .sup.32 P is commonly used by researchers in these fields to label genetic material (DNA/RNA) and proteins. Frequently, it is required to know the precise amount of a radioisotope contained in various test samples. Quantitative measurements of the amount of radioactive material present in a test sample usually are expressed as an activity in disintegrations per minute. These measurements provide valuable information both in preparing the radio-labelled chemicals and in measuring an amount of radio-labelled material recovered from a system under investigation. Measurements of the activity of a sample also are needed to insure the safety of personnel handling the radioisotopes.
At the present time, most measurements of activity are obtained from either scintillation counting or from Geiger counting. Scintillation counting uses photomultiplier tubes to detect photons produced in a scintillation medium in response to absorption by the medium of beta and gamma radiation. Many of the photons emitted from the scintillation medium are incident upon a photocathode of a multiplier phototube. These photons are converted to photoelectrons and are multiplied in number at a succession of phototube electrodes, called dynodes, the output of which is a measurable electrical pulse related to the incident radiation.
Liquid scintillation counting operates on the same basic principle as scintillation counting, except that the scintillation medium is a liquid into which is dissolved, suspended or otherwise intermixed the radioactive sample being tested. Radioactive emissions of a sample are measured by collecting photons emitted from the scintillation medium and generating photoelectrons responsive thereto to produce electrical pulses related to the incident beta and gamma radiation.
Scintillation and liquid scintillation counting require special sample preparation and the use of special sample containing vials in order to provide a quantitative measure of the amount of radioactive material present in a particular sample. Accordingly, an extra material handling step, involving a transfer of radioactive material into one of the special vials, is required when using these techniques. This transfer step is undesirable, for it is accompanied by an element of error in the measurement of material transferred to the vials. When this measurement error is added to the error inherent to the particular experiment or test technique being utilized, further uncertainty as to the accuracy of the quantitative data obtained from the sample results. Furthermore, the preparation of even a small amount of material for scintillation counting results in the loss of that material for further experimentation. In many cases, where only a very limited quantity of material is available, this loss may be unacceptable.
Several manufacturers produce sophisticated instruments for both beta and gamma radiation counting. Generally, these instruments are designed to count large numbers of radioactive samples in an automated mode. These instruments generally include many features such as multiple sample carriers capable of holding hundreds of individual samples, multi-user protocols for automatically altering data manipulation from one set of samples to the next, and advanced calibration and correction techniques for obtaining more accurate quantitative data. Needless to say, such devices are complex and expensive.
Geiger counters are generally used when counting small numbers of samples. These counters provide a simpler but much less reliable means for measuring an approximate activity of a radiation emitting sample. Geiger counters use gas filled tubes the contents of which are ionized by incident radiation to produce an electronic signal which registers on a meter or in an audio circuit. The magnitude of the electronic signal is proportional to the amount of radiation impinging upon the gas filled tubes. Commercial Geiger counters are generally hand held devices whose quantitative accuracy is limited by uncertainties in the geometrical positioning of the sample relative to the detector and the absence of careful calibration techniques. However, the instruments are very useful in determining the presence and/or location of radioactivity and in determining an approximate activity of the sample for safe handling considerations. Geiger counters are also helpful in assessing the progress of certain chemical reactions or experiments.
In most laboratories, and in most biotechnology laboratories in particular, the choice of radioactivity counting instrumentation is governed by the number of samples to be counted, the required accuracy of the results, the amount of sample availabe for analysis, and the availability of the instruments. Geiger counters are available in almost all laboratories which handle radioisotopes as a safety precaution for monitoring spilled or airborne radioactive materials. As a result, quick, approximate determinations at the laboratory bench are generally made using Geiger counters despite the limited degree of measurement accuracy they provide.
Scintillation or liquid scintillation counters are available in most laboratories only on a shared basis. Because of their relative size and cost, these instruments are generally located in one area which may be some distance from a researcher's workbench. Therefore, these instruments tend to be used only when a substantial number of samples have been accumulated for counting. Samples are generally loaded into an available test tube rack of an automatic sample handler, desired counting parameters are selected, and the samples are left in the counting machine to be counted in turn. Data obtained from the radiation counting is generally recorded by a printing device. The actual counting may occur many hours following the insertion of the samples into the machine and often the results are not available until the following day. While it is possible to count one or several samples in a scintillation or liquid scintillation counter, to do so is a tedious procedure which is infrequently undertaken. In order to count small numbers of samples, the current automatic sequence of the counting device must be interrupted, the samples inserted and counted, and then the instrument returned to the correct position in the automatic counting sequence. Errors in material handling and machine operation may result in lost counting time causing delays of many hours in obtaining the data from the automated runs with larger numbers of samples.